The present invention relates to oximetry sensors and, in particular, pulse oximetry sensors which include coded information relating to characteristics of the sensor.
Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which passes light through a portion of the patient""s tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have been provided with light sources and photodetectors that are adapted to operate at two different wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
An encoding mechanism is shown in U.S. Pat. No. 4,700,708, the disclosure of which is incorporated herein by reference. This mechanism relates to an optical oximeter probe which uses a pair of light emitting diodes (LEDs) to direct light through blood-perfused tissue, with a detector picking up light which has not been absorbed by the tissue. The operation depends upon knowing the wavelength of the LEDs. Since the wavelength of LEDs can vary, a coding resistor is placed in the probe with the value of the resistor corresponding to the actual wavelength of at least one of the LEDs. When the oximeter instrument is turned on, it first applies a current to the coding resistor and measures the voltage to determine the value of the resistor and thus the value of the wavelength of the LED in the probe.
U.S. Pat. No. 5,259,381 recognizes that the coded value of the wavelength of the red LED provided by a coding resistor may be inaccurate, since the actual wavelength can vary with temperature. Accordingly, this patent teaches including a temperature sensor in the oximeter probe to measure the actual temperature. With the actual temperature, and the coded wavelength value, a look-up table can be consulted to determine the actual LED wavelength for that temperature.
Another method of storing coded information regarding the characteristics of the LEDs is shown in U.S. Pat. No. 4,942,877. This patent discloses using an EPROM memory to store digital information, which can be provided in parallel or serially from the sensor probe to the remote oximeter.
Other examples of coding probe characteristics exist in other areas. Multiple calibration values are sometimes required, with this making the circuitry more complex or requiring many leads. In U.S. Pat. No. 4,446,715, assigned to Camino Laboratories, Inc., a number of resistors are used to provide coded information regarding the characteristics of a pressure transducer. U.S. Pat. No. 3,790,910 discloses another pressure transducer with a ROM storing characteristics of the individual transducer. U.S. Pat. No. 4,303,984 shows another probe with digital characterization information stored in a PROM, which is read serially using a shift register.
Typically, the coding element is mounted in the probe itself. For instance, U.S. Pat. No. 4,621,643 shows the coding resistor mounted in the probe element itself. In addition, U.S. Pat. No. 5,246,003 shows the coding resistor being formed with a printed conductive material on the probe itself.
In some devices, an electrical connector coupled by a cable to a device attached to a patient may include a coding element. For example, U.S. Pat. No. 3,720,199 shows an intra-aortic balloon catheter with a connector between the catheter and a console. The connector includes a resistor with a value chosen to reflect the volumetric displacement of the particular balloon. U.S. Pat. No. 4,684,245 discloses a fiberoptic catheter with a module between the fiberoptic and electrical wires connected to a processor. The module converts the light signals into electrical signals, and includes a memory storing calibration signals so the module and catheter can be disconnected from the processor and used with a different processor without requiring a recalibration.
In some applications, it would be desirable to provide multiple independent codes for different values using only two leads. This may allow backward compatibility, or compatibility with probes of other manufacturers. For example, Nellcor Puritan Bennett produces a probe with an encoded calibration resistor for providing a signal indicative of a known wavelength of a red LED, which signal upon being read by an oximeter allows the oximeter to select appropriate calibration coefficients for use in calculating arterial oxygen saturation. In order to modify such a Nellcor-type sensor to provide multiple independent codes, one approach is to use a different range of resistors to indicate a different characteristic. For example, Ohmeda Systems is believed to use a first range of resistors for their reusable sensors, and a second range of resistors for the disposable sensors. Thus, a single resistor essentially encodes in its most significant bit the sensor type, and then indicates the calibration curve to be used with the least significant bits of its value. Marquette is believed to produce a sensor and monitor which uses the presence of a resistance to indicate the type of sensor, with no resistance being present indicating a Marquette sensor. Other sensors are believed to read multiple values using three pins.
U.S. Pat. No. 5,645,059 teaches using a modulated signal to provide the coded data to a remote analyzer. U.S. Pat. No. 5,429,129 shows using a voltage regulator to produce a specific voltage value in response to an attempt to read by the analyzer.
The present invention provides an encoding element which is backward compatible and will provide a single coded value to older analyzers in response to a first signal, but will also provide a second coded value to a new analyzer or monitor. Both coded values are provided over the same first and second leads which are compatible with existing probes or sensors. The encoding element can be a resistor or other discrete component, a hybrid, a component group, an integrated circuit, or any other encoding mechanism.
In one embodiment, a first voltage level produces a first current from the encoding element, while a second voltage level will produce the second coded value. In one version of this embodiment, a zener diode is activated to connect a second resistance or to simply bypass a resistor when a high level voltage is provided exceeding the zener value. When the lower voltage is provided, the analyzer sees only the resistance it expects for the older type of sensor.
In a second embodiment, a first coded value is provided in response to a DC current from older analyzers. A second coded value is provided in response to an AC signal from newer analyzers. Yet another embodiment provides the coded value in the form of a resonant circuit in the AC driven mode. By varying the frequency applied, the frequency resulting in the highest voltage, indicating the resonant frequency, can be found. Thus, the coded value is provided in the value of the resonant frequency chosen.
In another embodiment, the second coded value is provided over the same two leads after a period of time, with the first value being provided prior to that period of time passing. If the period of time is chosen to correspond with that used by older analyzers, backward compatibility is ensured. In one embodiment, a thermistor is used which will heat up as current is applied over time, and eventually switch in a second coding resistor.
In yet another embodiment, a first coded value is provided in response to a DC current from older analyzers, and a second code is provided in response to an input digital code from newer analyzers over the same two leads.
In one embodiment, since some older analyzers may drive the calibration resistor with different polarities, the new coding element is compatible to provide the desired value regardless of the polarity of the driving signal to determine the first coded value.
In one embodiment, a third lead is used which is not used in prior art oximeters. Many prior art oximeters had positions for two additional pins which were not used. By using one of these pins, a control signal can be sent to switch which encoded value is provided across the original 2 leads. Alternately, three encoded values can be provided over the 3 leads.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with accompanying drawings.